The invention relates to a method for the plasma-nitriding of precipitation hardenable stainless steels or stainless maraging steels. The invention also relates to a shaver cap and a cutting device. The invention further relates to an electric shaver.
For years, maraging steels have been used in industry for applications where hardened steel was necessary. Old established methods for hardening steel, some dating back thousands of years such as heating and quenching, have been supplemented with more advanced methods, such as plasma nitriding, whereby nitrogen is included in the structure of the metal. This alteration of the structure of the metal yields a thin layer of hardened metal on the outside of the steel item, making it much more wear-resistant.
Given the many useful qualities of stainless steels, these have found wide application in all kinds of fields. Hardness, however, is not a particularly strong point of stainless steel. Hardening of stainless steel is compromised because of unwanted reactions, which do make the steel harder but also reduce its corrosion resistance.
To date maraging steel has mostly been employed in situations where hardness was a prime factor, but the corrosion resistance of maraging steels leaves room for improvement. A recent example of the dilemma between hardness and corrosion resistance proved to be the Coolskin Philishave®. This is an electric shaver that can be used for wet shaving, and was developed to combine the advantages of wet shaving with a razor with the safety features of an electric shaver. The shaver uses a shaver head with an outer blade made of very thin steel. Since the introduction of the Coolskin Philishave® there has been a problem in that the outer blade wears too fast. Because of the required hardness, this blade is made of maraging steel for want of a sufficiently hard stainless steel alternative, but this still proves to be insufficiently hard-wearing. At the same time it has become clear that improved corrosion resistance is also desired, and although above especially mentioned in relation to the Coolskin Philishave type of shaver it will be clear that improvement of the corrosion resistance of a cutting element of another type of shaver is also advantageous.
Further hardening of maraging steel according to the present state of the art has the disadvantage that the hardness can indeed be increased, but then the toughness decreases accordingly. In other words, hardened steel becomes brittle, making it unsuitable for certain purposes. It can be imagined that this problem is less acute in, say, ball bearings where the hardened surfaces are inflexible than in a shaver blade which is very thin (order of magnitude of 70 μm) and flexible. Increased hardening impairs the corrosion resistance of the maraging steel.
Steel exists in various crystalline states, that is to say the atoms can be arranged in different configurations. Also the addition of other elements can alter the atomic configuration of steel, and thereby its characteristics. Stainless steel, for example, is an alloy of steel with up to 18% chromium and around 11% nickel, giving this steel its stainless corrosion-resistant properties. Steel itself is harder than pure iron because it contains carbon. The main states of the steel of interest for the present invention are martensite and austenite. Of these, austenite is the softer, more deformable state. In general it can be said that items are shaped with the metal in the austenitic state and subsequently hardened by heating to transform the metal at least partly into the martensitic state. Traditionally the steel is quenched, i.e. rapidly cooled off, in order to maintain the martensitic state at a lower temperature. Another widely used method is precipitation hardening.
Several solutions have been proposed in the art to the problem of improving the hardness of certain stainless steels and non-stainless maraging steels. EP 1 008 659 discloses a method for the production of steel plates from a specific type of martensite hardening steel. The method disclosed teaches that the age hardening temperatures have to stay below the martensite/austenite transition temperature and that also the surface hardening is realized at temperatures below the martensite/austenite transition temperature. A drawback of the method known in the art is that this method can only be applied to iron-nickel-cobalt maraging steel, which is not corrosion-resistant.